Hot-rolled steel sheet and cold-rolled steel sheet and manufacturing method thereof

Abstract

A steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances and machine structures and of constructing buildings, and a manufacturing method thereof. are provided.The steel sheet is a hot-rolled steel sheet of carbon steel or low-alloy steel, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D (μm) at the depth of ¼ of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by the formulas (1) and (2) given below and the increase rate X (μm / min) in average ferrite crystal grain diameter at 700° C. at the depth of ¼ of the sheet thickness from the steel sheet surface and said average crystal grain diameter D (μm) satisfy the relation defined by the formula (3) given below:1.2≦D≦7   formula (1)D≦2.7+5000 / (5+350·C+40·Mn)2  formula (2)D·X≦0.1   formula (3)wherein C and Mn represent the contents (in % by mass) of the respective elements in the steel.

Description

TECHNICAL FIELD[0001]The present invention relates to a hot-rolled or cold-rolled steel sheet having ultra fine crystal grains, and to a manufacturing method thereof. More particularly, it relates to a hot-rolled or cold-rolled steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances and machine structures and of constructing buildings, and to a manufacturing method thereof.BACKGROUND ART[0002]The steel sheet to be used as a raw material for structural members in transport machines, typically automobiles, and various industrial machines is sometimes required not only to be excellent in such mechanical characteristics as strength, workability and toughness but also to have weldability in parts assembly and / or corrosion resistance during use. In order to generally improve the mechanical characteristics of a steel sheet, it is effective to render a stee...

AI Technical Summary

Benefits of technology

[0013]The present inventors made various investigations and experiments concerning the mechanical characteristics and thermal stability of the fine ferrite crystal grain microstructure and, as a result, found that, in order to attain both good mechanical characteristics and good thermal stability, it is most important (a) for the average ferrite crystal grain diameter to be kept within a certain specific range and (b) to set an upper limit to the product D·X (μm2 / min) of the increase rate X (μm / min) in average ferrite crystal grain diameter D (μm) at temperatures in the vicinity of about 700° C. just below the A1 point and that average crystal grain diameter D (μm). They also found that, in order to attain better thermal stability, it is preferable (c) to retain the ferrite crystal grain distribution within a certain specific range and / or for no strains resulting from rolling to be left within ferrite crystal grains. Further, they found that steel sheets obtained by taking such findings into consideration, when cold-rolled and then heat-treated, can again acquire a ferrite crystal grain microstructure thermally stable and fine in the same manner as mentioned above. Then, they made various investigations and experiments in search of (d) a novel method of producing hot-rolled or cold-rolled steel sheets having such a microstructure and such characteristics. Furthermore, as regards welded members, they found (e) that, in fusion welding, it is preferable to specify the hardness balance in the weld and (i) that, in resistance welding, it is preferable to strive for attaining a hardness balance and preventing embrittlement in the weld.

[0185]The steel sheet according to the present invention has ultra fine crystal grains and is excellent in thermal stability, hence can endure the heat during welding or plating, and also is excellent in mechanical characteristics. Such steel sheet excellent in thermal stability and mechanical characteristics can be produced with ease by the method according to the present invention.

Problems solved by technology

However, even when steel sheets having a fine crystal microstructure are obtained by such methods, the microstructure is low in thermal stability.

Therefore, even if the microstructure is rendered fine and the mechanical characteristics are improved, the subsequent step of welding of steel sheets or hot-dip plating of steel sheets readily results in coarsening of crystal grains due to heating in that step and the mechanical characteristics of the sheets are seriously impaired; this is another problem.

Further, when these hot-rolled steel sheets are subjected to cold rolling and heat treatment to give thin steel sheets, a further problem arises, namely the heat treatment readily results in coarsening of crystal grains and cold-rolled steel sheets having a fine microstructure can scarcely be obtained.

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